1. Field of the Invention
This invention relates to fluid flow measuring apparatus. More particularly, this invention relates to such apparatus of the vortex-shedding type employed to measure the velocity of liquid or gas flow through a pipe.
2. Description of the Prior Art
It has been known for many years that vortices are developed in a fluid flowing past a non-streamlined obstruction. It also has been known that with certain arrangements the vortices are developed by alternately shedding at regular intervals from opposite edges of the obstruction to form corresponding rows of vortices. Such vortices establish a so-called von Karman "vortex street", which is a stable vortex formation consisting of two nearly-parallel rows of evenly-spaced vortices travelling with the flow stream.
In a von Karman vortex street, the vortices of one row are staggered relative to those of the other row by approximately one-half the distance between consecutive vortices in the same row. The spacing between successive vortices in each row is very nearly constant over a range of flow rates, so that the frequency of vortex formation is correspondingly proportional to the velocity of the fluid. Thus, by sensing the frequency of vortex shedding it is possible to measure the fluid flow rate. Devices for that purpose are often referred to as vortex meters.
Various types of vortex meters have been available commercially for a number of years. Such meters basically comprise a vortex-shedding body mounted in a flow tube together with a sensor for detecting the vortex pressure fluctuations. Commonly, the vortex shedding body has a blunt surface facing the oncoming fluid. One type of sensor which has been highly successful employs a piezoelectric crystal to detect the vortex pressure fluctuations and to produce corresponding electrical pulses. Preferably, such crystal is mounted in a sealed oil-filled cavity which receives the pressure fluctuations through flexible metallic diaphragms. A particularly suitable arrangement of this type is disclosed in U.S. Pat. No. 4,085,614 (Curran et al).
The piezo-electric sensor shown in FIG. 6 of the above Curran et al patent is located directly behind the vortex-shedding surfaces of the blunt body which, as is usual, extends perpendicularly across the full diameter of the flow tube. Although this configuration has found substantial commercial success, it is primarily suitable for relatively large-sized meters, e.g. having flow tubes 2" in diameter and above. For small-diameter flowmeters, i.e. so-called small line-sized meters having flow tubes of about 1.5" or less in diameter, it clearly is preferable to mount the sensor outside of the flow tube.
It has previously been proposed to mount at least portions of a vortex sensor outside of the flow tube. For example, in FIG. 13 of the above Curran et al patent, the piezo-electric crystal element is located outside of the flow tube, and receives pressure pulses through capillary-sized conduits connected to respective diaphragm-sealed chambers in the interior of the vortex-shedding body. Such an arrangement however has been found not to be fully satisfactory, especially because the capillary conduits attenuate the pressure pulse signals sufficiently to prevent high-level performance.
U.S. Pat. No. 3,722,273 provides another showing of a vortex meter having a sensor element located outside of the flow tube. A thermal "hot-wire" sensor element receives fluid pressure signals through small conduits connected to opposite sides of a chamber formed inside the vortex-shedding body. Other sensor configurations of that general type are disclosed in FIGS. 16-18 of U.S. Pat. No. 3,777,563. The sensor arrangements described in the latter two patents however suffer from important practical disadvantages making them unacceptable as solutions to the problem addressed by the present invention.